Electrochemical storage devices according to the present invention are storage devices operated at high temperatures, which have an operating temperature level of at least 100° C. In particular, such electrochemical storage devices have an operating temperature level of between 200° C. and 350° C. Operating temperatures of up to 500° C. are however also feasible. Such electrochemical storage devices are embodied in particular using sodium-nickel chloride cell (NaNiCl2 cell) technology or are sodium-sulfur cells (NaS cells).
When operating such electrochemical storage devices, it is necessary to dissipate the heat arising during electrical charging or discharging. At the same time it is necessary to ensure that the electrochemical storage device may be kept at the operating temperature level during operation, since only at such temperatures does the solid electrolyte have an ion conductivity suitable for operation and can the storage device remain free from harmful thermal stresses. The operating temperature level thus sometimes also decisively determines the electrical behavior of the electrochemical storage device.
To establish a suitable operating temperature level, such electrochemical storage devices are generally brought into external thermal contact with a heat transfer medium, such that heat can be suitably supplied or dissipated. Heat supply and/or dissipation may here be undertaken by a sensor assisted heat management system, which ensures that a suitable operating temperature range can always be maintained. In the event of exposure of the electrochemical storage device to relatively high current densities or in the event of higher current density current consumption, an increased amount of heat is formed compared with the normal charging or discharging process. To dissipate this heat efficiently during operation of the electrochemical storage device, it is usual to provide a current density limit, which may also be suitably adjusted depending on the state of charge or discharge of the cell. To a limited extent, it is even briefly also possible to allow current densities elevated beyond this limit during charge or discharge, insofar as the amount of heat generated in the process is sufficiently low in relation to the thermal capacity of the electrochemical storage device.
However, the use of such electrochemical storage devices in particular for mobile applications increasingly requires design for elevated power densities. Even for stationary applications for intermediate storage of electrical energy for instance from a power supply network these requirements are also significantly increased. Such requirements may also present a major challenge to the heat management system, which has to ensure cooling of the electrochemical storage device.
To be able efficiently to dissipate the heat released in the electrochemical storage device during such operation, the applicant has proposed in a parallel application to make the spacing between the solid electrolyte and the wall surrounding it so small that even in the event of a low electrochemical storage device charge this space determined by the spacing is completely filled with anode material. As a result of this filling it is possible for the electrochemical storage device to influence the released heat very positively by direct heat outflow via the anode material to the outside at the side part of the electrochemical storage device. By way of example, in such an electrochemical storage device the cell can comprising the side part may be of tapered construction, wherein the ceramic solid electrolyte is wetted completely by anode material due to the small spacing between the wall of the can and the surface thereof even in the case of a low state of charge. As a result of this geometry, a uniform temperature distribution around the solid electrolyte may at the same time be achieved, which in turn results in a reduction in mechanical stresses in the solid electrolyte. This extends the service life of the solid electrolyte and thus of the entire electrochemical storage device.
If for example such electrochemical storage devices are operated at an elevated power density, an elevated amount of waste heat also arises, which has typically to be dissipated outwards via the side part (dissipation via the top part or the bottom part is however in principle also feasible). The elevated operating temperature in the electrochemical storage devices also accelerates the reaction rates of undesired chemical reactions within the storage device. Such chemical reactions are for example liable to reduce the capacity of the cathode or promote corrosion and degradation phenomena of the electrodes and walls. In this respect, it is necessary for undisturbed operation to dissipate this sometimes large amount of heat sufficiently quickly from the storage device to reduce such influences which for instance shorten the service life of the storage device.